Nowadays information technology is confronted with an increasing amount of data, due to more complex or multimedia applications. Accordingly, removable data storage devices with a high storage capacity are needed, e.g. for high resolution movies or video games. Well at the beginning of information technology, magnetic storage devices were favored, while nowadays, optical storage media such as CD (Compact Disk), DVD (Digital Versatile Disk) or BD (Blu-Ray Disk) are dominating the market for removable data storage media.
Optical data storage is generally limited by the optical resolution of the read/write-system. Straightforward methods of increasing the optical resolution involve widening of the focused beam and opening angle, i.e. the numerical aperture NA, at the costs of lens complexity. Further approaches are narrowing the allowable tilt margins for the optical recording media or reducing the wavelength of the scanning laser into the blue or near-UV range. A different approach for reducing the focus spot size in an optical data storage system is using near-field optics with a high numerical aperture. This high numerical aperture is generally achieved by help of a solid immersion lens (SIL). While conventional systems like CD, DVD or BD operate in the optical far-field regime, which is described by classical optics, the aforementioned new systems work in the optical near-field regime, which is described by near-field optics. For conventional systems the working distance, i.e. the air gap between the surface of the optical storage medium and the first optical surface of the read/write-head, is in the scale of 100 μm. In contrast, systems making use of near-field optics need a very small working distance or air gap, which is in the scale of 50 nm. An optical storage system for recording and/or reading making use of near-field optics is disclosed in WO 2005/104109 A1.
To provide compatibility between different storage media, so called compatible drives are needed. These drives are capable of reading from and/or writing to storage media belonging to different media generations, e.g. Blu-Ray disks as well as near-field optical storage media. Consequently, the optical head of such a compatible drive needs to be able to work in the far-field regime as well as in the near-field regime. A known approach is to include two objective lenses in the optical head of the respective data storage apparatus. The first objective lens is used in the far-field regime, whereas the second objective lens is used in the near-field regime. Each objective lens is designed for its specific purpose. However, such optical heads are usually fragile and expensive.
An alternative solution is disclosed in JP 2000-163792. An optical head includes an objective lens that is composed of a lens and an optical unit. The optical unit has a correction board and a solid immersion lens. For near-field operation the lens and the optical unit are arranged in the beam path. For far-field operation the optical unit is moved out of the beam path.